1. Field of the Invention
The present invention relates to atomic clocks and, more particularly, to a thermally-insulated micro-fabricated atomic clock structure and a method of forming the atomic clock structure.
2. Description of the Related Art
A planar coil is a coil where each loop of the coil lies within the same plane. A current flowing in a planar coil generates a magnetic field that is perpendicular to the plane. When an object with a magnetic moment is placed in a magnetic field, the magnetic field exerts a force on the magnetic moment that tries to align the magnetic moment with the direction of the magnetic field.
Individual electrons have an intrinsic magnetic moment which can be thought of in the same manner as the magnetic moment that results from a current in a planar coil. As a result, when an electron is placed in a magnetic field, the magnetic field tries to align the intrinsic magnetic moment of the electron with the direction of the magnetic field.
Individual electrons also have an intrinsic angular momentum that is associated with the intrinsic magnetic moment. The interaction of the intrinsic angular momentum with the alignment force of the magnetic field causes the intrinsic magnetic moment of the electron to precess about the direction of the magnetic field. This precession is analogous to a spinning top as the top wobbles. The intrinsic magnetic moment of an electron precessing about the direction of an applied magnetic field is at an angular frequency known as the Larmor frequency.
The Larmor frequency can be used as a standard to maintain the frequency of a clock. The clock, which is commonly known as an atomic clock, oscillates at the Larmor frequency. In addition, the clock periodically determines the Larmor frequency, and uses the determined Larmor frequency to correct any drift in the oscillation frequency of the clock.
Atomic clocks which utilize the Larmor frequency as the frequency standard typically include a vapor cell, a vertical cavity surface emitting laser (VCSEL), and a photodiode. The vapor cell, which lies in an externally generated magnetic field, contains a gas that includes alkali atoms and buffer atoms.
Alkali atoms have a single electron in the outer s subshell of the atom. For example, rubidium87 has a single electron in the 5 s subshell of the fifth shell, while cesium has a single electron in the 6 s subshell. In the absence of a magnetic field, the s subshell has two energy levels known as hyperfine energy levels. However, in the presence of a magnetic field, the subshell has a number of energy levels known as Zeeman sublevels within the hyperfine energy levels.
The alkali atoms within the gas are commonly implemented with, for example, 85Rb atoms, 87Rb atoms, K, or Cs atoms. The buffer atoms within the gas, which are utilized to reduce collisions between the alkali atoms and the inner surface of the vapor cell, are commonly implemented with, for example, N2 atoms.
The light output by the VCSEL is tuned to a frequency which, when circularly polarized (and after having been linearly polarized by either a linear polarizing element or if the VCSEL is designed to produce linear polarized light), is absorbed by the single electrons in the outer shells of the alkali atoms in the gas. For example, the VCSEL can be tuned to output light with a wavelength of 794.8 nm which, after being circularly polarized, is absorbed by the single electrons in the outer shells of the 87 Rb atoms. The VCSEL can alternately be turned to output light with a wavelength of 894.35 nm which, after being circularly polarized, is absorbed by the single electrons in the outer shells of the Cs atoms.
If the single electron in the outer shell of an alkali atom absorbs right-hand circularly polarized light, then the electron transitions from the s subshell to either the outer p subshell, while the projection number M of the electron is always raised by +1. When the output light is removed, the single electron emits a photon in a random direction, and falls back to one of the Zeeman sublevels within the hyperfine energy levels of the s subshell. The state the electron falls to is exactly defined by the quantum selection rules.
When the electron falls back, the projection number M of the electron also changes by −1, 0, or +1 but in a random manner. Thus, when a number of such events occur to the same electron, each time the electron goes to a higher state, the projection number M of the electron is always raised by +1. However, as the electron falls down to the ground state, the projection number M of the electron on average does not change.
As a result, the electron will eventually land on the highest M level in the ground state. In the gasses under consideration, both the ground state S1/2 and the elevated state P1/2 (or P3/2) have the same number for M levels. Thus, when the electron reaches the highest M level in the g round state, the electron cannot be pumped because there is not a higher M level in the excited state.
To again reabsorb light, the population in the ground state M levels has to be de-pumped. Additional energy (magnetic or optical) must be supplied to the electron at the Larmor frequency. The additional energy at the Larmor frequency causes the electron in the highest ground state M level to drop to a lower M level that is associated with the outer shell where the electron can again absorb light energy.
The photons that pass out of the vapor cell include a non-absorption component, which represents the light output by the VCSEL that was not absorbed by the electrons in the outer shell of the gas within the vapor cell, and an emission component, which represents the photons that are randomly emitted by the falling electrons. The photodiode detects these photons, and generates an output signal that has both a non-absorption component and an emission component.
One common approach to adding additional energy at the Larmor frequency is the Bell-Bloom (BB) technique. In the BB technique, the light output by the VCSEL is modulated by a frequency that is swept across a range of frequencies. When the light output by the VCSEL is frequency modulated at the Larmor frequency, the electrons drop to a lower energy level and begin reabsorbing light energy, which causes a noticeable dip in the intensity of light received by the photo detector.
Thus, the Larmor frequency can be determined by determining the modulated frequency that caused the intensity of the received light to dip. The detected Larmor frequency is then used to correct any drift in the frequency oscillation of the clock, thereby ensuring that the clock oscillates at the Larmor frequency.
Two of the drawbacks of conventional Larmor-based atomic clocks are size and cost, which then limit the types of applications where atomic clocks can be commercially utilized. In response to these drawbacks, micro-fabricated atomic clocks have been proposed which can be mass produced in conventional integrated circuit fabrication facilities.
However, many of the applications for micro-fabricated atomic clocks require the clock to operate with very little power in an environment where the external temperature can range from, for example, −40° C. to +100° C. This is difficult to achieve because the VCSEL and the gas within the vapor cell must each be heated to operate within specific temperature ranges to ensure proper operation.
Thus, there is a need for a micro-fabricated atomic clock which can operate with very little power in an environment where the external temperature can drop to −40° C., while at the same time maintaining the temperature required for the proper operation of the VCSEL and the gas within the vapor cell.